In the construction of motor vehicle bodyworks, hot or cold-rolled, surface-refined steel sheets are used for reasons of corrosion protection. Sheets of this type are subject to numerous requirements. They have, on the one hand, to be readily deformable and, on the other hand, to have high strength. The high strength is achieved by the addition to the iron of specific alloy constituents such as Mn, Si, Al and Cr. In order to optimize the property profile of steels of this type, it is conventional to anneal the sheets immediately prior to the coating with zinc and/or aluminum in the molten bath. Whereas the melt dip coating of steel strips containing merely low contents of the aforementioned alloy constituents is unproblematic, the melt dip coating of steel sheet having higher alloy contents presents difficulties. On the surface of the steel sheet, there result defects in the adhesion of the coating, and uncoated points even form.
In the prior art, there have been a large number of attempts to avoid these difficulties. However, there does not yet appear to have been an optimum solution to the problem.
In a known process for melt dip coating a steel strip with zinc, the strip to be coated passes through a directly heated preheater (direct fired furnace—DFF). In the gas burners used, changing the gas/air mixture can result in an increase in the oxidation potential in the atmosphere surrounding the strip. The increased oxygen potential leads to oxidation of the iron on the surface of the strip. The iron oxide layer thus formed is reduced in a subsequent furnace stretch. Purposeful adjustment of the thickness of the oxide layer at the surface of the strip is very difficult. It is thinner at high strip speed than it is at low strip speed. A clearly defined composition of the surface of the strip therefore cannot be produced in the reductive atmosphere. Again, this can lead to problems of adhesion of the coating to the surface of the strip.
In contrast to the above-described known system, modern melt dip coating lines comprising an RTF (radiant tube furnace) preheater do not use gas-heated burners. The iron therefore cannot be pre-oxidized by changing the gas/air mixture. Instead, in these systems, the complete annealing treatment of the strip is carried out in an inert gas atmosphere. However, during such annealing treatment of a steel strip comprising relatively high alloy constituents, these alloy constituents can diffuse to the surface of the strip, where they form non-reducible oxides. These oxides prevent optimum coating with zinc and/or aluminum in the molten bath.
The patent literature discloses various processes for melt dip coating a steel strip with various coating materials.
DE 689 12 243 T2 discloses a process for continuous hot dip coating a steel strip with aluminum, wherein the strip is heated in a continuous furnace. In a first zone, surface impurities are removed. For this purpose, the furnace atmosphere has a very high temperature. However, as the strip passes through this zone at high speed, it is heated merely to approximately half the atmospheric temperature. In the subsequent second zone, which is under inert gas, the strip is heated to the temperature of the coating material, aluminum.
DE 695 07 977 T2 discloses a two-stage process for hot dip coating a steel alloy strip containing chromium, wherein the strip is annealed in a first stage to obtain iron enrichment at the surface of the strip. Subsequently, the strip is heated in a non-oxidizing atmosphere to the temperature of the coating metal.
It is known from JP 02285057 A to hot dip galvanize a steel strip in a multiple-stage process. For this purpose, the previously cleansed strip is treated in a non-oxidizing atmosphere at a temperature of approximately 820° C. The strip is then treated at approximately 400° C. to 700° C. in a mildly oxidizing atmosphere before it is reduced at its surface in a reductive atmosphere. Subsequently, the strip, cooled to approximately 420° C. to 500° C., is hot dip galvanized in the conventional manner.